Passive optical network system and ranging system thereof

ABSTRACT

A station-side communication device connected to subscriber-side communication devices via an optical combining device; sending, to the subscriber-side communication devices, a distance measurement request signal; computing transmission delay times of optical signals from the individual subscriber-side communication devices by receiving distance measurement signals, and including: a threshold control part identifying the level of distance measurement signals; a signal detection part detecting breaks in the distance measurement signals from the threshold control part; a transmission granting part determining the timing at which transmission of optical signals is granted, and a reset timing generation part that, there is notification of detection of a break in the distance measurement signal from the signal detection part while it is being notified that distance measurement is carried out to and from the subscriber-side communication devices from the transmission granting part, sends a reset signal indicating that the voltage level is reset to the threshold control part.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2006-279446 filed on Oct. 13, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present application pertains to a PON (Passive Optical Network)system in which a plurality of subscriber connection devices share anoptical transmission line.

As an optical access system, there is known the PON which makes a 1-to-nconnection (n being an integer equal to or greater than 2) between anOLT (Optical Line Terminal) arranged on the station side and an ONU(Optical Network Unit) arranged on the subscriber side, by means of adevice passively carrying out combining of optical signals, such as anoptical splitter. A plurality of ONUs are connected to the terminals(e.g. PCs or the like) of the respective subscribers, the electricalsignals from a terminal being converted into optical signals andtransmitted toward the OLT. The optical splitter having received opticalsignals from the plurality of ONUs optically multiplexes (time division)the same optical signals. Inversely, an optical signal from the OLT issplit, by means of the optical splitter, into a plurality of opticalsignals and transmitted toward a plurality of ONUs, each ONU selectivelyreceiving and processing signals, from among the transmitted destinedfor it.

As mentioned above, an uplink signal transmitted from a plurality ofONUs toward an OLT is time division multiplexed by means of an opticalsplitter. The OLT determines/notifies the transmission timing of theoptical signals with respect to the respective ONUs so that the opticalsignals from the plurality of ONUs do not collide, each ONU transmittingsequentially the optical signals at the notified timing. As specified inCh. 8 and Ch. 9 of ITU-T Recommendation G.984.1, since the optical fiberlength is e.g. set arbitrarily to one range from among the ranges 0-20km, 20-40 km, and 40-60 km for each ONU, the distances between the OLTand the ONUs, i.e. the optical fiber lengths, are not necessarily equal,also leading to a difference in the transmission delay times of theoptical signals transmitted from each ONU toward the OLT. Consequently,there is a need for the OLT to determine the transmission timing of theoptical signals by taking into account the optical signal transmissiondelay times arising from the difference in the distance of each ONU.

In order to implement this, the OLT uses so-called ranging which isdescribed in Ch. 10 of ITU-T Recommendation G.984.3, and by means ofthis, the OLT adjusts the transmission timing of the respective ONUs asif each ONU had been installed at equal distances, and the opticalsignals from a plurality of ONUs are made not to mutually interfere. Inother words, the OLT assumes that all the ONUs are separated by just theidentically same distance, determines/notifies the timing at which eachONU transmits an optical signal, and the OLT further notifies each ONUof the optical signal delay time arising from the difference between theconcerned assumed distance and the distance at which each ONU isactually installed, and each ONU transmits an optical signal at a timingdelayed, from the transmission timing notified from the OLT, by just thenotified delay time.

In ranging, it is necessary for the OLT to transmit a signal formeasuring the distance with respect to the ONU. When the ONU returns thedistance measurement frame, the OLT receives the same signal, measuresthe time from the request for transmission of the signal for distancemeasurement until the reception of the signal for distance measurement,i.e. the roundtrip delay time, and finds out how much the ONU isseparated from the OLT. Next, the OLT, in order to make all the ONUsappear to be at an equal distance, sends instructions for each ONU todelay transmission by just a time called the equalization delay (EqD).E.g., in order to make all the ONUs have a roundtrip delay time of 20km, it indicates to the ONU an equalization delay equal to (“20 kmroundtrip delay time”)-(“measured roundtrip delay time”). The ONU isprovided with a circuit that transmits data with a fixed delay of justthe indicated equalization delay, and by means of the aforementionedinstruction, and uplink data transmission is carried out so that all theONUs have a 20 km roundtrip delay time.

Also, in the Ethernet™ PON system defined in Ch. 64 of the IEEE 802.3Standard, the aforementioned distance measurement is carried outnotwithstanding the fact that no equalization delay instruction ispresent. Instead, after the distance measurement, in case the OLT sendsa grant to the ONU, the Start value of the grant is compensated on thebasis of the measured roundtrip delay time.

SUMMARY OF THE INVENTION

In ranging, the signal for distance measurement sent by the OLT isreceived by a plurality of ONUs, and each ONU having received thissignal transmits to the OLT a response signal with respect to the samesignal. Since the timing at which the response signals from each ONU atthis point in time arrive at the OLT is not adjusted, there is thepossibility that the OLT receives a number of response signals within ashort time period. In order to prevent this, there is provided in theOLT, for a certain fixed time interval after a signal for distancemeasurement has been transmitted to the ONU, a non-signal domain(ranging window) devised not to receive any response signal other thanthe response signal first received.

In this way, the OLT transmits a signal for distance measurement, sincethere is computed for each ONU an equalization delay time for the sameONU, it is not possible, within the ranging window of one ONU, for otherONUs to transmit an optical signal with respect to the OLT. For thisreason, in order to carry out ranging of an ONU located within the rangeof e.g. a fiber length of 0-60 km, a non-signal domain (ranging window)with a length of 600 μs corresponding to a 60 km roundtrip delay time isnecessary. As mentioned above, since the distance measurement is carriedout with respect to the ONUs within one ranging window, for e.g. thedistance measurement of e.g. 128 ONUs, there is needed an extendednon-signal domain of 76.8 ms, lengthened 128 times. Further, when thestability of the system is taken into account, it is desirable for thedistance measurement to take an average over a plurality of times, andif the ranging is e.g. carried out using an average of four times thedistance measurement result, the domains which cannot be utilized by theuser are further lengthened by a factor 4, there being necessary anextended non-signal domain of 307.2 ms.

In order to suppress the loss of signal domain due to this ranging, itis acceptable to broaden the interval during which ranging isimplemented and to make the loss uplink domain sufficiently small. E.g.in the aforementioned example, if the ranging interval is chosen to be30 seconds, the portion occupied by the 307.2 ms extended non-signaldomain becomes on the order of 1%, something which can be thoroughlyneglected. However, in this case, there occurs the new problem that 30seconds are required to activate all the ONUs at once. If the importanceof communications service is taken into consideration, it is desirable,in order to suppress to the utmost the service interruption timeresulting from a temporary failure, for the time to activate all theONUs at once have a sufficiently small value, e.g. 1 second.

The present invention has for an object to provide an OLT, ONU, and PONsystem making possible the activation of one hundred or more ONUs whilemaking efficient use of the band, and in a short time.

In order to combine a low band loss and a short activation time, it isacceptable to enable distance measurements of a plurality of ONUs insideone ranging window.

The aforementioned task is implemented by means of a method wherein theOLT is provided with a plurality of distance measurement circuits,receives a plurality of distance measurement signals transmitted fromthe plurality of ONUs inside one interval of the ranging window,validates a delimiter detection circuit directly after receiving adistance measurement signal, and carries out a reset of an automaticthreshold circuit.

Also, as another means for attaining the task, the OLT periodicallygenerates, regardless of the presence of an uplink ranging response inthe ranging window, plurality of ATC reset pulses, and when the ONUreceives a ranging request, it transmits a plurality of rangingresponses at an interval which is different from an integer multiple ofthe aforementioned period. By transmitting a plurality of rangingresponses, at least one signal is able to bring the distance measurementto success without colliding with the ATC reset pulse.

According to the present invention, it is possible to provide an opticalaccess system which, while making effective use of the band, enables theactivation of one hundred ONUs or more in a short time, within 1 second.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an embodiment of a PON network configurationaccording to the present invention.

FIG. 2 is a diagram showing an embodiment of a downlink PON signalframe.

FIG. 3 is a diagram showing an embodiment of an uplink PON signal frame.

FIG. 4 is a diagram showing an embodiment of an OLT functional block.

FIG. 5 is a diagram showing an embodiment a functional block related toranging processing.

FIG. 6 is a diagram showing an embodiment of an optical signal receivingportion.

FIG. 7 is a diagram showing an embodiment of a functional blocksupplying an ATC reset.

FIG. 8 is a diagram showing a time chart of the first embodiment.

FIG. 9 is a diagram showing a time chart of the second embodiment.

FIG. 10 is a diagram showing a time chart of the third embodiment.

FIG. 11 is a diagram showing a time chart of the fourth embodiment.

FIG. 12 is a diagram showing the operation of an optical signalreceiving portion of an OLT.

FIG. 13 is a diagram showing an example of ranging operation in a PON.

FIG. 14 is a diagram showing an embodiment of the hardware configurationof an OLT.

DESCRIPTION OF THE EMBODIMENTS

Below, embodiments of the present invention will be explained.

First Embodiment

FIG. 1 shows the configuration of an optical network in which thepresent invention is applied.

PON 10 is composed of an optical splitter 100, an OLT 200 being a deviceon the station side installed in an office building of atelecommunications operator or the like, a trunk fiber 110 connectingOLT 200 and the optical splitter, a plurality of ONUs 300 beingsubscriber side devices installed inside the respective subscriberresidences or in the vicinity thereof, and a plurality of branch fibers120 respectively connecting optical splitter 100 and a plurality of ONUs300. OLT 200 can be connected, via trunk fiber 110, optical splitter100, and branch fibers 120, to e.g. 32 ONUs 300. Also, user terminalssuch as telephones 400 and Personal Computers 410 are respectivelyconnected to the plurality of ONUs 300. PON 10 is connected via OLT 200to a PSTN (Public Switched Telephone Network) or the Internet 20 andtransmits and receives data to/from external networks.

In FIG. 1, five ONUs are illustrated which respectively have differingfiber lengths from OLT 200. In FIG. 1, ONU 300-1 has a fiber length fromOLT 200 of 1 km, ONU 300-2 has a fiber length from OLT 200 of 10 km, ONU300-3 has a fiber length from OLT 200 of 20 km, ONU 300-4 has a fiberlength from OLT 200 of 10 km, and ONU 300-n has a fiber length from OLT200 of 15 km. In a signal 130 transmitted in the downlink direction fromOLT 200 to ONU 300, the respective signals destined for ONU 300 are timedivision multiplexed. Each ONU 300 receives signal 130, determineswhether or not the signal is destined for it, and moreover, in case thesignal was one destined for it, delivers the signal to a telephone 400or a Personal Computer 410, based on the recipient of the signal.

Also, in the uplink direction from ONU 300 to OLT 200, signal 150-1transmitted from ONU 300-1, signal 150-2 transmitted from ONU 300-2,signal 150-3 transmitted from ONU 300-3, signal 150-4 transmitted fromONU 300-4, and signal 150-n transmitted from ONU 300-n, become a signal140 after having passed optical splitter 100 and being time divisionmultiplexed, and reach OLT 200. Since OLT 200 understands in advancefrom which ONU a signal was received at which timing, it carries outprocessing by identifying the signal from each ONU in response to thereceived timing.

In FIG. 2, there is shown an example of a downlink PON signal frametransmitted from OLT 200 to each ONU 300. The downlink frame is composedof a frame synchronization pattern 201, a PLOAM domain 202, a grantinstruction domain 203, and a frame payload 204. Grant instructiondomain 203 corresponds to a so-called “US Bandwidth MAP” shown inSection 8.1.3.6 of the same Recommendation, and the OLT specifies theuplink transmission grant timing of each ONU, using this domain. The “USBandwidth MAP” domain comprises a “Start” value designating thebeginning of the transmission grant and an “End” value designating itscompletion, with a designation of the respective byte units beingcarried out. This value, having the meaning of granting transmission, isalso called a grant value.

Further, in the individual ONUs, a plurality of band allocation unitscalled T-CONT (Trail CONTainer) can be allocated, the designations ofuplink and downlink transmission grant timing being carried out for eachT-CONT. In grant instruction domain 203, there are stored, for eachT-CONT, a “Start” value expressing the timing at which the opticalsignal transmission starts and an “End” value expressing the timing atwhich the optical signal transmission ends. T-CONT is a band allocationunit in DBA and, in case e.g. the ONU has a plurality of buffers, T-CONTIDs which are identification information items concerning T-CONT, aregiven to the respective buffers, it also being possible to control foreach buffer from the OLT.

The “ranging time” message in FIG. 13 which will be subsequentlydescribed is stored in PLOAM domain 202 and a “ranging request” signal310-1 and a “grant, request report” signal 320 including information asto the timing at which transmission of optical signals start in each ONUare stored in “Grant” instruction domain 203. In frame payload 204, usersignals and the like from OLT 200 toward ONU 300 are stored. Details aredescribed in ITU-T Recommendation G.984.3.

In FIG. 3, there is shown an example of an uplink PON frame transmittedfrom an ONU to the OLT. Uplink signal 150-1 coming from ONU 300-1 iscomposed of a preamble domain 301, a delimiter domain 302, a PLOAMdomain 303, a queue length domain 304, and a frame payload 305. Theaforementioned “Start” value indicates the start position of PLOAMdomain 303 and “End” value 313 indicates the end position of framepayload 305. Immediately before each uplink signal, there is set a guardtime for preventing a collision with the previous burst signal. Thedifference between the aforementioned “End” value and the subsequent“Start” value corresponds to the guard time which is a domain of nouplink signal. In other words, the time from the end position of framepayload 305 of the uplink signal until preamble domain 301 of thesubsequent uplink signal corresponds to the guard time. Further, in thepresent embodiment, by detecting the signal of the delimiter domain, itis identified that the data from the delimiter domain onward are newdata. In other words, the delimiter domain is used as information foridentifying breaks between signals.

In FIG. 4, a configuration example of an OLT 200 according to thepresent invention is shown. An ONU transmission and reception part 401is a part transmitting and receiving optical signals to and from ONU 300which carries out processing such as converting the optical signalreceived from the ONU into an electrical signal by means of a opticalsignal reception processing part 403, transmitting as optical signalsthe electrical signal inside the device by means of an optical signaltransmission processing part 404, and transmitting the signal to theONU. Network transmission and reception part 402 carries outtransmission and reception of PSTN or Internet 20 signals to and fromhigher-level networks. A control part 409 carries processing and thelike according to the PON Protocol with respect to input and outputsignals. A received signal processing part 405 carries out processingsuch as cutting and dividing electrical signals received from opticalsignal reception processing part 403 into PON frames. A rangingprocessing part 406 carries out ranging processing to be subsequentlydescribed. Transmission granting part 407 sets the “Start” value and the“End” value of each ONU, from the values for the communication bandsallocated to each ONU by means of DBA processing, and notifies each ONUof these values. A transmitted signal processing part 408 generates PONframes transmitted to each ONU.

In FIG. 14, there is shown an example of a hardware configuration of OLT200. OLT 200 has a control board 1400 managing the operation of thewhole device and a plurality of network interface boards 1440, 1450, and1460 being respectively connected to the network and carrying out signaltransmission and reception. Control board 1400 has a memory 1410 and aCPU 1420 and controls each network interface board through a hub 1430.Each network interface board has an ONU transmission and reception part401 and a network transmission and reception part 402 as well as a CPU1470, carrying out processing required for the transmission andreception of signals occurring between the ONU and the Internet or aPSTN, and a memory 1480. A wide variety of processing types occurring inthe present embodiment function e.g. by CPU 1470 executing programsstored in memory 1480. Otherwise, as the need arises, dedicated hardware(LSI and the like) specialized in each type of processing is available,it being acceptable to execute processing by means of this. Further, theconfiguration of the OLT hardware is not limited hereto, it beingacceptable for various devices to carry out the processing in responseto an appropriate need.

In FIG. 13, there are shown the ranging signals occurring in the opticalaccess network of the present embodiment. OLT 200 transmits a “rangingrequest” signal 310-1 toward ONU 300-1. ONU 300-1, after receiving“ranging request” signal 310-1, transmits a “ranging response” signal311-1 after a determined fixed time. OLT 200 determines, from thedifference in the transmission timing of “ranging request” signal 310-1and the reception timing of “ranging response” signal 311-1, thedistance to ONU 300-1. Next, OLT 200 transmits a “ranging time” message312-1 and sets an equalization delay 330-1 with respect to ONU 300-1. Bythe functioning of this equalization delay 330-1, regardless of thephysical installation position of ONU 300-1, the distance from OLT 200is regulated as if it were 20 km. Below, the distance measurement of ONU300-2 and ONU 300-3 is carried out in the same way.

After this, OLT 200 requests, by transmitting “grant and request report”signal 320 with respect to ONU 300-1, ONU 300-2, and ONU 300-3, that therequested transmission volume be notified together with giving an uplinktransmission grant. Corresponding to this signal, ONU 300-1 transmitsuser data and a report 321-1. In the report, the volume of uplinksignals waiting for transmission inside ONU 300-1 is displayed with thenumber of bytes and notified to OLT 200. The transmission of user dataand report 321-1 is carried out after receiving the user data and report321-1 with a timing delayed, from the instructed timing 331-1 based onthe grant, by just equalization delay 330-1. The transmission control ofONU 300-2 and ONU 300-3 is also similar, so by means of this operation,when OLT 200 receives an uplink signal, the user data from ONU 300-1 andreport 321-1, the user data from ONU 300-2 and report 321-2, and theuser data from ONU 300-3 and report 321-3, are lined up efficientlywithout mutually colliding or being greatly separated and are receivedby OLT 200. In this way, on the basis of the respective transmissionrequests of ONU 300-1, ONU 300-2, and ONU 300-3, Dynamic BandwidthAllocation (DBA) is implemented by means of changing the volume ofuplink transmission requests.

In case a plurality of ONU distance measurements are implemented withinone ranging window, in order that the length of the preamble signal forsynchronization permitted in the uplink burst signal defined in ITU-TRecommendation G.984.3 does not exceed several bytes, a manipulationapplying a reset to the receiver by means of a timing known in advanceis indispensable for carrying out the pullback of the identificationthreshold value of the uplink signal and the clock with a short preamblelike this. In practice, in a steady state after ONU activation, sincethe arrival time of the uplink burst signal is controlled with the OLTinstruction, applying a reset to the uplink receiver is simple. However,in the ranging process, since the arrival times of the distancemeasurement signals differ depending on the distance between the OLT andthe ONU, it is not possible to apply a reset to the receiver by means ofa timing known in advance. If it is an Ethernet™ PON defined in the IEEE802.3ah standard in which a preamble with a length of several hundredbytes is permitted, signal reception is possible using fast-tracking AGC(Automatic Gain Control) and not using a reset, but in the shortpreamble defined in Recommendation G.984.3, no method is proposed forcarrying out a distance measurement of a burst signal of several byteswithin one ranging window. Since this is implemented in the presentembodiment, an improvement is added to OLT ranging processing.

Using FIG. 5, an explanation will be given of the details of thefunctional block related to the ranging processing of the OLT 200 in thepresent embodiment. An optical signal received from trunk fiber 110 isconverted into an electrical signal by an O/E conversion part 501 andthere is carried out an identification of a value “0” or a value “1” onthe basis of an appropriate threshold value in an ATC (AutomaticThreshold Control) unit 503. Subsequently, clock extraction and retimingare carried out, a delimiter detection part 504 detecting delimiterdomain 302 shown in FIG. 3 and identifying a break in the uplink signal.PON frame decomposition part 505 decomposes the uplink PON frameexplained in FIG. 3 and sends the queue length report stored in queuelength domain 304 to a grant generation part 509. Also, a distancemeasurement part 507 implements the distance measurement occurring inthe ranging operation explained in FIG. 13 and computes the equalizationdelay for each ONU. Grant generation part 509 carries out DBA processingusing the queue length report from the PON frame decomposition part,determines the communication band allocated to each ONU, and generates a“Start” value and an “End” value. Moreover, this “Start” value and this“End” value are handed over to a reset timing part 506 and are also usedfor the reset of ATC 208. A PON frame generation part 510 stores thesignal from grand generation part 509, based on the downlink PON framesignal format explained in FIG. 2, in grant instruction domain 203 andtransmits it. Also, the equalization delay computed by distancemeasurement part 507 is also stored in the “Ranging time” message formatby PON frame generation part 510 and is transmitted toward each ONU. Adriver 511 converts the electrical signal from PON frame generation part510 from a voltage to a current signal and E/O conversion part 502converts the current signal into an optical signal and transmits it totrunk fiber 110.

FIG. 6 shows a configuration example of the optical signal receptionportion of the OLT in the present invention. Inside O/E conversion part501, an APD (Avalanche Photo Diode) connected to a high-voltage biassource 601 is given a reverse bias at a high voltage and the receivedoptical signal is amplified and converted into a current signal by meansof the avalanche effect. The converted current is converted into voltagewith a TIA (Trans Impedance Amplifier) 244 composed of a resistance 604and an amplifier 605. Together with the voltage of the received signalbeing output in digital form with an A/D converter, in ATC 503, athreshold value is set at half amplitude and a signal identified ashaving a value “0” or a value “1” is output. The output of an amplifier606 has a peak detection carried out on it using the diode function fromthe base to the emitter of a transistor 607, is held in a capacitor 608,and is provided as the threshold value of an amplifier 609. Immediatelybefore the reception of signals from each ONU, a reset signal isprovided to transistor 609 and the threshold value held in capacitor 608is discharged and reset to the “0” level.

The operation of ATC at the time of implementing a distance measurementof a plurality of ONUs within one ranging window is explained by meansof FIG. 12. The length of the preamble signal for synchronizationpermitted in the uplink burst signal defined in ITU-T RecommendationG.984.3 does not exceed several bytes. To carry out the pullback of theuplink signal identification threshold value and the clock pullback witha short preamble such as this, a circuit called ATC (Automatic ThresholdControl) shown in FIG. 6 is made necessary. ATC 503 detects theamplitude of the received signal for each input burst at high speed, andby inputting the same threshold value in the capacitor and holding itthere, it is stable even for data with consecutive 0's and can receive.The reverse face thereof is that, as shown in FIG. 12, a manipulation ofapplying a reset to ATC 503 by means of a timing known in advance isindispensable after the burst signal has ended. If there is no reset,the threshold value is left held at the value of the previous signal, soif subsequently a smaller signal is received, the threshold value is toobig and correct signal identification is not carried out.

Especially at the time of ranging, since the signal is returned at anearlier time and with larger amplitude the closer the ONU is, it isnormal for the subsequently received signal to have an amplitude whichgradually gets smaller. Since, in the steady state after ONU activation,the arrival time of the uplink burst signal is controlled by OLTassignment, it is easy to apply a reset to the aforementioned receiver.Since, however, in the ranging process, the arrival times differ for thedistance measurement signals depending on the distance between the OLTand the ONU, it is not possible to apply a reset to the receiver bymeans of a timing known in advance and there is a need for determining atiming at which OLT 200 applies a reset to ATC 503.

In FIG. 7, there is shown a block diagram of a reset timing generationpart 506 supplying a reset signal to ATC 503. A start edge detector 701receives a “Start” value/“End” value form grant generation part 509 andwhen it is time to start reception of optical signals from the ONU, astart edge signal is generated. The present signal is used for delimiterdetection validation in a normal operating state after the ONU has beenactivated and ATC reset.

On the other hand, the start and end timing of the ranging window, whichis output by grant generation part 509, is notified to a periodic timinggeneration part 702, or during the interval of the ranging window, bymaking the signal be put in the ON state, or the like, a ranging windowsignal indicating the interval of the ranging window is input. Further,there is also input, into periodic timing generation part 702, adelimiter detection notification signal indicating that a delimiterwhich is output by delimiter detection part 504 and included in thesignal from the ONU has been detected. Periodic timing generation part702 generates a delimiter detection validation signal which validatesthe processing of delimiter detection part 504 and hastens the carryingout of new delimiter detection processing and an ATC reset signal forresetting ATC 503, if a timing detection notification signal is inputwhile it is being indicated by means of a ranging window signal thatthere is currently a ranging window interval.

The fact of validating delimiter detection part 504 together with theresetting of ATC 503 means there is a possibility, if delimiterdetection is carried out continuously, of erroneously recognizing as thedelimiter a signal inside the payload, being random data, in a distancemeasurement signal. In order to prevent an erroneous recognition such asthis, once delimiter detection part 504 detects a delimiter, thefollowing delimiter detection operation is temporarily halted. Whendelimiter detection part 504 receives a delimiter detection validationsignal, delimiter detection starts for the second time. In this way,even if there is currently a ranging window interval, by resetting ATC503 whenever a delimiter signal from a different ONU is detected, itbecomes possible to receive and process “ranging request” signals from aplurality of ONUs even within one ranging window. Further, after the ATCreset, reception of the following distance measurement signal becomesprecisely possible. Since the distance measurement signal has as far aspossible a length of several tens of seconds, the probability that adistance measurement signal from a different distance collides is smalland several ONU distance measurements can be carried out within oneranging window.

Logical summing part 703 merges and outputs the delimiter detectionvalidation and ATC reset signal in the aforementioned normal operatingstate coming from start edge detector 701, the aforementioned delimiterdetection validation signal within the aforementioned ranging window andthe ATC reset signal coming from periodic timing generation part 702,and validates delimiter detection part 504 together with resetting ATC503.

In FIG. 8, a time chart of the ranging processing of the presentembodiment is shown. A signal generated by periodic timing generationpart 702 explained in FIG. 7 is indicated as ATC reset 802. OLT 200transmits toward ONU 300-1, ONU 300-2, and ONU 300-3 distancemeasurement requests (“ranging requests”) 804, ONU 300-1, ONU 300-2, andONU 300-3 respectively independently generating random delays 805, 806,and 807 and transmitting distance measurement signals (“rangingresponses”). In case ONUs with nearly equal distances are present withinone PON interval, by giving random delays defined in Ch. 10 ofRecommendation G.984.3 and making send distance measurement signals, thearrival times at the OLT are randomized, so the collision of distancemeasurement signals are stochastically avoided, making it possible tobring to success a plurality of ONU distance measurements within oneranging window.

OLT 200 carries out ATC reset 802-1 at the start position of a rangingwindow 808 by means of periodic timing generation part 702 and resetsthe threshold value of a preceding burst signal 803. Next, OLT 200receives a first distance measurement signal 809, immediately thereaftercarries out an ATC reset 802-2, and resets the threshold value of firstdistance measurement signal 809. Further, OLT 200 receives a seconddistance measurement signal 810, immediately thereafter carries out anATC reset 802-3, and resets the threshold value of second distancemeasurement signal 810. Next, OLT 200 receives a third distancemeasurement signal 811, immediately thereafter carries out an ATC reset802-4, and resets the threshold value of third distance measurementsignal 811.

In this way, by receiving a distance measurement signal and immediatelythereafter carrying out an ATC reset and the validation of a delimiterdetection circuit, it is possible to bring a plurality of distancemeasurements to success within one ranging window.

Second Embodiment

As another embodiment, there may be considered one in which periodictiming generation part 702 periodically generates and outputs, duringthe ranging window interval, ATC resets and delimiter detectionvalidation signals. Even in this case, it is possible for periodictiming generation part 702, by means of a ranging window signal receivedfrom grant generation part 509, to output the aforementioned resetsignals and the like periodically just during the ranging windowinterval.

In FIG. 9, the time chart of the present embodiment is shown. Thesignals generated by periodic timing generation part 702 are indicatedas ATC resets 902. OLT 200 transmits distance measurement requests(ranging requests) 905 toward ONU 300-1, ONU 300-2, and ONU 300-3, andONU 300-1, ONU 300-2, and ONU 300-3 respectively transmit distancemeasurement signals (ranging responses). Specifically, ONU 300-1transmits a distance measurement signal 910, ONU 300-2 transmits adistance measurement signal 911, and ONU 300-3 transmits a distancemeasurement signal 912.

OLT 200 carries out an ATC reset 902-1 at the start position of theranging window and resets the threshold value of a preceding burstsignal 904. Next, OLT 200 periodically carries out ATC resets 902-2,902-3, 902-4, 902-5, 902-6, and 902-7 at equal intervals 903. Similarlyto the example shown in FIG. 9, if distance measurement signals fromeach ONU can be received in the ATC reset interval, ranging processingwith respect to a plurality of ONUs within one ranging window becomespossible.

Third Embodiment

In the method of the second embodiment, there is the possibility that anATC reset and a distance measurement signal from an ONU collide, so atthis point, the ranging processing with respect to the ONU would fail.As yet another embodiment, there may be considered a method devised sothat each ONU having received a ranging request from OLT 200 replieswith a plurality of ranging requests, leaving intervals in between.

In FIG. 10, the time chart of the present embodiment is shown. Regardingportions which are the same, as in the time chart of FIG. 9, the samereference numerals are attached. A signal generated by periodic timinggeneration part 702 is shown as ATC reset 902. OLT 200 transmitsdistance measurement requests (ranging requests) 905 toward ONU 300-1,ONU 300-2, and ONU 300-3. ONU 300-1, ONU 300-2, and ONU 300-3respectively transmit a plurality of distance measurement signals(ranging responses). Specifically, ONU 300-1 transmits a distancemeasurement signal 910-2 leaving an interval 906-1, after havingtransmitted distance measurement signal 910-1, and further transmits adistance measurement signal 910-3 after an interval 906-2. In the sameway, ONU 300-2 transmits a distance measurement signal 911-1 and, afterleaving an interval 907-1, transmits distance measurement signal 911-2and, further, after interval 907-2, transmits distance measurementsignal 911-3. ONU 300-3 also transmits distance measurement signals912-1, 912-2, and 912-3 with intervals 908-1 and 908-2 sandwiched inbetween.

From among the distance measurement signals 910-1, 910-2, and 910-3transmitted from ONU 300-1, distance measurement signal 910-1 isnormally received after ATC reset 902-1. Distance measurement signal910-2 collides with ATC reset 902-2, so reception fails. Distancemeasurement signal 910-3, since distance measurement signal 902-2 andATC reset 910-2 have collided, has a reset which is insufficient, butsince the immediately preceding signal has been transmitted from thesame ONU 300-1 and is a signal with the same amplitude, there is apossibility that it is received normally, even if the threshold value isinsufficient. In this way, from among the three times the distancemeasurement signal was sent, it is received normally at least once.

In the same way, from among the distance measurement signals 911-1,911-2, and 911-3 transmitted from ONU 300-2, distance measurement signal911-1 is received normally after ATC reset 902-3, but distancemeasurement signal 911-3 collides with distance measurement signal 912-2from another ONU 300-3 and reception fails. Even here, from among thethree times the distance measurement signal is sent, it is receivednormally at least once. Further, from among the distance measurementsignals 912-1, 912-2, and 912-3 transmitted from ONU 300-3, distancemeasurement signal 912-1 collides with ATC reset 902-5 and receptionfails and as for distance measurement signal 912-2, it collides withdistance measurement signal 911-3 from another ONU 300-2, so receptionfails, but distance measurement signal 912-3 is received normally afterATC reset 902-7. Either way, from among the three times the distancemeasurement signal was sent, the signal is received normally at leastonce.

By having the respective ONUs transmit a plurality of distancemeasurement signals, it is avoided, although the total number ofdistance measurement signals rises and the probability increases thatthere will somewhere occur a collision, that the same ONUs are combinedand collisions occur repeatedly, by changing the number of intervals ofthe plurality of distance measurement signals transmitted by therespective ONUs, so there is a high probability that at least once thedistance measurement will succeed. In other words, taking interval 906between transmissions of distance measurement signals by ONU 300-1,interval 907 of ONU 300-2, and interval 908 of ONU 300-3 to bedifferent, and further, even if ONU 300-1 is the same, it is possible toreduce the probability of collision by taking intervals 906-1 and 906-2to be different.

The intervals with which ONU 300-1 transmits distance measurementsignals may be set autonomously by the ONU by using a serial number oran ONU-ID indicated from the OLT, or the OLT may indicate a distancemeasurement signal interval with respect to the ONU. Alternatively, itis also possible to use dynamically changing random values as intervalsof the plurality of distance measurement signals and in this case also,the ONU may be provided with a random counter and autonomously set theintervals, or the OLT may be provided with the random counter andindicate the intervals of the distance measurement signals with respectto the ONUs.

Fourth Embodiment

In the methods of the aforementioned second and third embodiments, incase ONUs with nearly equal distances are present in one PON interval,it is possible, by conferring random delays defined in e.g. Ch. 10 ofRecommendation G.984.3 and making transmit distance measurement signals,to randomize arrival times to the OLT and to stochastically avoidcollision of distance measurement signals and so to bring to success aplurality of ONU distance measurements within one ranging window.However, since collision prevention based on random delays is astochastic avoidance measure, it is good in the case of a small numberof ONUs, but in case several hundred multiple ONUs are simultaneouslycarrying out distance measurements, there is a possibility that therewill somewhere occur a collision when attempting distance measurements.An explanation will be given of an embodiment which, in order to avoidthis situation, restricts the number of ONUs returning distancemeasurement signals by means of SN mask generation part 508 in the poststage of delimiter detection part 507 shown in FIG. 5.

In ITU-T Recommendation G.983.1, there are defined a method wherein theOLT carries out a distance measurement after indicating one eight bytelong serial number being an individual ONU identification number; and amethod wherein the OLT, without indicating a serial number, after havingrequested a signal for distance measurement, and when detecting acollision of signals from a plurality of ONUs, requests a signal fordistance measurement for a second time while indicating a part of theserial number, and gradually makes adjustments so that only one signalfor distance measurement is transmitted. Also, in a GPON, there is inaddition to these methods defined a mechanism called random delaywherein the ONU adds a random time delay and transmits a signal fordistance measurement; and a method wherein first an ONU serial number isobtained from the signal for distance measurement received at first withrandom delay, and subsequently, using the obtained serial number, adistance measurement is carried out after indicating one ONU. In thepresent embodiment as well, there is proposed a method in which, usingthe serial number given to each ONU, the number of ONUs replying to aranging request message from the OLT is restricted.

In the present embodiment, if the need arises because distancemeasurement signal collisions occur frequently or the like, the SN maskgeneration part 508 shown in FIG. 5 restricts the serial numbers of ONUsreturning a distance measurement signal. Further, as for the conditionof activating SN mask generation part 508, it may be devised so that thecontrol part controlling the operation of OLT 200 as a whole detectsfrequent occurrences of collisions and notifies SN mask generation part508 of the activation, or it may be devised so that distance measurementpart 507 counts the number of distance measurement failures and requestsan activation when the counted number of failures exceeds apredetermined value. Alternatively, the condition may be devised so thatSN mask generation part 508 itself counts the number of distancemeasurement failures and judges whether an activation is necessary ornot by comparing the number against a predetermined value.

In FIG. 11, the time chart in the present embodiment is shown. OLT 200transmits a distance measurement request (ranging request) 1101 towardONU 300-1, ONU 300-2, and ONU 300-3. If ONU 300-1 and ONU 300-2respectively transmit distance measurement signals (ranging responses)1102-1 and 1102-2, collisions occur, and a distance measurement failureis detected by means of a CRC error of the received signal in OLT 200.In the prior art, all the information on a distance measurement signalreceived in the case where an OLT had failed in a distance measurementwas discarded, but in the present embodiment, at least the informationincluding the serial number of the ONU is accumulated temporarily, fromamong the received distance measurement signals. The place of storingthe serial number may be memory space of OLT 200, or generation part 508may be devised to hold the serial numbers.

Next, SN mask generation part 508 of OLT 200 extracts the first half ofthe serial number from the aforementioned accumulated distancemeasurement signals and outputs it to PON frame generation part 510. PONframe generation part 510 creates a serial number mask message 1104,described in Ch. 9 of ITU-T Recommendation G.984.3, so as to match theconcerned extracted value and transmits it toward the ONU. Serial numbermask message 1104 is used to make only those ONUs react whose eight-byteserial numbers with a part masked coincide. Even if it is assumed thatdistance measurement signals from a plurality of ONUs collide and thedistance measurement fails, there is a high probability that it has beenpossible to correctly receive part of the serial numbers. Consequently,if the ONUs are narrowed down to those whose serial numbers match inpart and a ranging request 1105 is transmitted, the reactions of e.g.ONU 300-2 and ONU 300-3 are masked, so the probability that distancemeasurement signal 1106 of only ONU 300-1 can be received becomeshigher.

If distance measurement fails here as well, if OLT 200 transmits aranging request 1108 after serial number mask generation part 508 hasextracted the value of the first quarter of the serial number in aserial number mask message 1107 and narrowed down the ONUs, thereactions of ONU 300-2 and ONU 300-3 are masked, so the probability ofbeing able to receive distance measurement signal 1109 of ONU 300-1 onlyis further increased. In this example, the first half or the firstquarter of the serial number is extracted and a narrowing down of theONUs is implemented, but it is acceptable for SN mask generation part508 to use an arbitrary position of the serial number to conduct thenarrowing down.

In the aforementioned embodiment, an explanation was given in accordancewith a GPON compliant with ITU-T Recommendation G.984.3, but it can alsobe applied to other PON systems, e.g. an Ethernet™ PON system defined inCh. 64 of the IEEE 802.3 Standard.

In this way, even if it is not possible, in the present embodiment, toobtain the entire eight byte length of a serial number due to acollision, there is used a serial number mask message, defined inRecommendation G.984.3, in which the first half (four bytes) of aneight-byte serial number is input, the ranging candidate ONUs arelimited, and a distance measurement is carried out for a second time.Even in the case where a collision of a plurality of distancemeasurement signals is generated, the probability that a part of thefirst half of distance measurement signal can be received normally ishigh if a random delay function is used in combination, it is possible,by limiting the ONUs by means of the serial number mask message, tobring a distance measurement from the limited ONUs to success with ahigh probability. Even in this processing, it is still possible, in casethere occurs a collision of distance measurement signals, to furtheronly use the first quarter (two bytes) of the serial number, use aserial number mask message, and attempt a distance measurement for asecond time. As for the serial number limitation methods, a number ofthem can be considered, such as the method of reducing by one bit at atime, the method of progressively increasing the number of bits to bereduced from 1 to 2, 4, etc., and the method of using a random lengthfor each distance measurement, but the method of gradually decreasingthe length of the used serial number from 1 to ½, ¼, ⅛, etc., has thebest balance between comprehensiveness and efficiency.

Further, the present embodiment is not limited to the second and thirdembodiments, since it may be used in combination with the firstembodiment.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A station-side communication device connected to a plurality ofsubscriber-side communication devices via an optical combining device;sending, to said plurality of subscriber-side communication devices, adistance measurement request signal for measuring the distances to saidrespective plural subscriber-side communication devices; computingtransmission delay times of optical signals from said individualsubscriber-side communication devices by receiving distance measurementsignals which are replies from said plurality of subscriber-sidecommunication devices with respect to said distance measurement requestsignal, and comprising: a threshold control part identifying the voltagelevel of said distance measurement signals converted from opticalsignals into electrical signals; a signal detection part detectingbreaks in said distance measurement signals from said threshold controlpart; a transmission granting part determining the timing at whichtransmission of optical signals is granted, with respect to saidindividual subscriber-side communication device; and a reset timinggeneration part that, if it is notified that there has been detected abreak in said distance measurement signals from said signal detectionpart while it is being notified from said transmission granting partthat distance measurement is carried out to and from saidsubscriber-side communication devices, sends a reset signal indicatingthat said voltage level is reset to said threshold control part.
 2. Thestation-side communication device according to claim 1, wherein saidreset timing generation part sends a signal indicating that breaks insaid distance measurement signal are detected, together with said resetsignal, to said signal detection part.
 3. A station-side communicationdevice connected to a plurality of subscriber-side communication devicesvia an optical combining device; sending, to said plurality ofsubscriber-side communication devices, a distance measurement requestsignal for measuring the distances to said respective pluralsubscriber-side communication devices; computing transmission delaytimes of optical signals from said individual subscriber-sidecommunication devices by receiving distance measurement signals whichare replies from said plurality of subscriber-side communication deviceswith respect to said distance measurement request signal, andcomprising: a threshold control part identifying the voltage level ofsaid distance measurement signals converted from optical signals intoelectrical signals; a signal detection part detecting breaks in saiddistance measurement signals from said threshold control part; atransmission granting part determining the timing at which transmissionof optical signals is granted, with respect to said individualsubscriber-side communication device; and a reset timing generation partthat, while it is being notified from said transmission granting partthat distance measurement is carried out to and from saidsubscriber-side communication devices, sends a plurality of resetsignals, indicating that said voltage level is reset, to said thresholdcontrol part.
 4. The station-side communication device according toclaim 3, wherein the plurality of reset signals sent by said resettiming generation part are sent periodically.
 5. The station-sidecommunication device according to claim 3, further comprising a serialnumber mask generation part designating at least a part of the serialnumber allocated dedicatedly in said subscriber-side communicationdevice, said serial number mask generation part limiting, by notifyingsaid plurality of subscriber-side communication devices of the serialnumbers designated by said serial number mask generation part, saidsubscriber-side communication devices transmitting said distancemeasurement signals to said subscriber-side communication devices thatinclude said designated serial numbers.
 6. The station-sidecommunication device according to claim 5, wherein at least a part ofthe serial numbers designated by said serial number mask generation partis at least a part of the serial numbers included in said distancemeasurement signals for which it has not been possible to receive allthe signals, due to the fact of colliding with other signals.
 7. Thestation-side communication device according to claim 6, wherein saidserial number mask generation part specifies the serial number using thefirst half of the serial number and further, in case said station-sidecommunication device has failed a distance measurement, specifies aserial number using the first quarter of the serial number.
 8. Anoptical communication system connecting a station-side communicationdevice and a plurality of subscriber-side communication devices via anoptical combining device; transmitting to said plurality ofsubscriber-side devices a distance measurement request signal formeasuring distances from said station-side communication devices andrespectively to said plural subscriber-side communication devices; and,by receiving distance measurement signals which are replies with respectto said distance measurement request signal requested by saidstation-side communication device to said plurality of subscriber-sidecommunication devices, computing the transmission delay times of opticalsignals from said individual subscriber-side communication devices,wherein said station-side communication device comprises: a thresholdcontrol part identifying the voltage level of said distance measurementsignals converted from optical signals into electrical signals; a signaldetection part detecting breaks in said distance measurement signalsfrom said threshold control part; a transmission granting partdetermining the timing at which the transmission of optical signals isgranted, with respect to said individual subscriber-side communicationdevices; and a reset timing generation part which, while it is notifiedfrom said transmission granting part that distance measurements arecarried out to and from said subscriber-side communication devices,sends a plurality of reset signals indicating that the voltage level hasbeen reset in said threshold control part, said subscriber-sidecommunication devices returning a plurality of said distance measurementsignals, if said distance measurement request signals are received fromsaid station-side communication device.
 9. The optical communicationsystem according to claim 8, wherein said plurality of reset signalssent by said reset timing generation part are sent periodically.
 10. Theoptical communication system according to claim 9, wherein: saidsubscriber-side communication devices periodically return a plurality ofdistance measurement signals; and the period of said reset signals sentby said reset timing generation part and the period of said distancemeasurement signals returned by said subscriber-side communicationdevices are mutually different.
 11. The optical communication systemaccording to claim 8, wherein said station-side communication devicecomprises a means of storing identification numbers attached to saidplurality of subscriber-side communication devices, determines theperiod with which said plurality of distance measurement signals withrespect to said individual subscriber-side communication devices arereturned one by one on the basis of said identification numbers, andnotifies said respective subscriber-side communication devices of saiddetermined period.
 12. The optical communication system according toclaim 8, wherein said station-side communication device has acalculation means outputting random values, and notifies said individualsubscriber-side communication devices of the random values output bysaid calculation means, taking the period with which said plurality ofdistance measurement signals are returned one by one.
 13. The opticalcommunication system according to claim 11, wherein said opticalcommunication system is a PON (Passive Optical Network), and saidstation-side communication device notifies said subscriber-sidecommunication devices of said period, using the downlink PON frame grantinstruction domain.
 14. The optical communication system according toclaim 12, wherein said optical communication system is a PON (PassiveOptical Network), and said station-side communication device uses thedownlink PON frame grant instruction domain to notify saidsubscriber-side communication devices of said period.